Rocket engine combustion chambers and nozzles are typically exposed to high velocity gases at temperatures in excess of 3300° C. (6000° F.) for long periods. To cool the walls closely exposed to the hot gas, closely spaced channels are typically provided in the walls for coolant flow. The channels are connected at the channel ends and are supplied with coolant from a manifold. The coolant circulates in the channels to cool the wall material during operation.
The channels can be manufactured by a number of well known methods. One such method entails using a machining tool to machine the cooling channels on the exterior of the combustion chamber or nozzle liner. Channel machining tools typically operate a rotating spindle that includes one or more slot cutting blades that cut one or more channels concurrently. The machining tool provides spindle rotation of the slot cutting blades and special movement thereof along a desired channel profile. The depth of the cutting blade can be adjusted manually by an operator or automatically by the machining tool based on numerically defined profile.
Because of the very harsh thermal environment encountered by rocket engine combustion chambers, the remaining wall thickness at the bottom of the channel being cut can be as small as 0.020 inches to adequately cool the wall exposure to the hot gas. On the large diameter chambers or nozzles, the small remaining wall thickness results in a very flexible structure that does retain its intended reference shape during the channel cutting operation. When a channel machining tool is cutting channels that extend from the forward section to the aft section of the part, flexibility of the inner wall may cause the cutting blades to cut the channels deeper than a desired depth resulting in the remaining wall thickness being too small. Other factors, such as movement of the part on its mandrel during channel machining may also cause the channels to be cut deeper than desired. As a result, the undersize wall thickness may render combustion chamber or nozzle unusable.